Discharge lamps for operation from AC current which are usually used for projection apparatuses, for example a high-pressure mercury lamp sold by the applicant under the type designation P-VIP for use in video projectors, projection television sets and other projection applications, usually have to be subjected to burn-in at the end of the production process. For this purpose, small tips are grown on the electrode heads by operation for the first time, that is to say that tungsten deposition and rearrangement processes are effected at the arc attachment region. The tip growth is dependent on the frequency of the lamp current, inter alia.
High-pressure mercury lamps for projection applications (P-VIP) are operated from electronic ballasts (EBs, PT-VIP lamp drivers) which output a temporally variable AC current to the lamp. The mean current (IRMS) is controlled by the EB depending on the measured lamp voltage such that a preset electrical power (target value) is output to the lamp during operation. Depending on the operating mode, this power can be varied, for example by the projector or user.
The temporal form of the current profile (waveform) is dependent on a projector color wheel and/or the operating mode of the projector, for example, wherein the waveform consists of individual segments of different lengths, each of which assumes a specific momentarily constant lamp current intensity. Commutations can be inserted between the segments by the electronic ballast; that is to say that the direction of flow of the lamp current reverses its direction at these points. This results in different frequency components of the electric lamp operating current overall for each waveform.
The lamp voltage usually drifts during the lifetime of the lamp because the geometry and surface constitution and the thermal conditions associated therewith change at the electrode or the tip thereof. A burn-back or vaporization of the tips is typical; the consequence thereof is a decrease in luminous flux since the adaptation to the optical system, in particular to the reflector and to further light guiding elements of the projection apparatus, becomes less favorable as a result of the arc inevitably becoming longer. A further known phenomenon is the spontaneous melting of one of the electrode tips on account of a thermal overload resulting from radiation reflected back from the projector optical system, for example visible light reflected at a color wheel or else radiation components in the UV range.
Excessive tip growth is also possible as a result of unfavorable conditions, as a result of which the lamp assumes a very low running voltage. As a consequence thereof, this can result in the effect that the setpoint power can no longer be achieved on the basis of the dimensioning of the electronic ballast. The consequence thereof is an excessively low luminous flux.
Such great tip growth can be prevented for example by the electrode tips being melted in a targeted manner. In this context, WO 2010/086222 A1 discloses a method for operating a gas discharge lamp including a gas discharge lamp burner and a first and a second electrode, wherein the electrodes, before the first start-up thereof, have a nominal electrode spacing in the gas discharge lamp burner which is correlated with the lamp voltage, including the following steps: a) checking whether a blocking time corresponding to the time duration between two DC voltage phases has elapsed, b) if the blocking time has elapsed, applying DC voltage phases, or applying pseudo-commutations, for a predetermined time duration dependent on the lamp voltage, in such a way that a time duration for omitting commutations is predetermined for each lamp voltage.
Furthermore, asymmetrical waveforms can also be used. If the commutations are positioned in a waveform such that both very short and very long segment durations occur, then proportions of low and also high frequencies result for the lamp current in the frequency spectrum. In this context, DE 10 2011 089 592 A1 discloses a DLP projector for projecting at least one frame onto a projection surface. In this case, a discharge lamp is driven with a current waveform having a first and a second region, wherein a first frequency is assigned to the first region and a second frequency is assigned to the second region. The first frequency is calculated as f1=1/(2*T1), wherein T1 relates to the time period between the first and the second commutations. The second frequency is calculated as
            f      2        =          n      /              (                  2          *                                    ∑                              i                =                1                            n                        ⁢                          T              i                                      )              ,wherein Ti relates to the time durations from one commutation to the next commutation within the region. Furthermore, a modulation factor relating to the ratio of second frequency to first frequency is at least 3.
WO 2009107019 A2 discloses an operating procedure which determines a target value for the voltage by measuring the voltage at defined points in time. In the event of the target value being exceeded or undershot, a switch-over of an operating parameter takes place (for example of the lamp frequency or a change of current pulse heights). However, this involves switching merely between two discrete operating modes which differ with regard to their frequency or their current pulse heights, for example. What is disadvantageous about this is that the frequency or the current pulse heights often cannot be chosen freely, but rather are coupled to boundary conditions which are predefined by the color wheel, for example.